TRIPOLI, Jan 18 (Reuters) - Libyan war planes attacked targets in the restive south on Saturday after gunmen stormed an air force base and the government ordered in ground troops following days of skirmishes between rival tribesmen and militias.

Western powers fear the OPEC producer could slide into further instability as the government struggles to contain heavily-armed militias, tribesmen and Islamists who helped to topple Muammar Gaddafi in 2011 but refuse to disarm.

A lack of border controls and the ineffectiveness of a small army lacking equipment have turned Libya into a weapons smuggling route for al Qaeda in sub-Saharan countries and also a corridor for Islamist fighters heading to Syria and economic migrants heading to Europe.

Prime Minister Ali Zeidan went on national television to announce he had ordered troops to be sent to the south after a group of gunmen entered the Tamahind air force base outside Sabha, 770 km (480 miles) south of the capital Tripoli.

Defence Ministry spokesman Abdul-Raziq al-Shabahi said later that government forces had regained control of the base after air strikes.

"A force was readied, then aircraft moved and took off and dealt with the targets," he told reporters in Tripoli.

He gave no details of the military operation but blamed forces loyal to Gaddafi for the violence. Sabha, the biggest city in the south, has seen days of clashes between rival militias and tribesmen.

"The situation in the south ... opened a chance for some criminals ... loyal to the Gaddafi regime to exploit this and to attack the Tamahind air force base," he said. "We will protect the revolution and Libyan people."

After Zeidan's announcement, the General National Congress (GNC) assembly voted to put the army on general alert, state news agency Lana and a deputy said.

"There are some hostile movements against the (Libyan) revolution in some cities," said lawmaker Salah Ajauda, a member of the GNC's security committee.

In Tripoli, security forces were also been put on maximum alert, a security spokesman said.

ITALIANS KIDNAPPED

In the volatile east, a security source said two Italian construction workers had been kidnapped in Derna, east of Benghazi, where they had been staying in a cement factory.

"There was a group of Libyan construction workers waiting for them on the highway east of Derna to fix a hole in the road, but the Italians did not arrive," the source said. "We are trying to establish the identity of the kidnappers, to find out about their demands."

Derna is a stronghold of radical Islamists. Residents temporarily blocked a road outside the town to protest against the kidnapping, a local activist said.

Benghazi, the main city in the east, has been rocked by a wave of assassinations of army and police officers as well as car bombs. Most Western nationals left the city after the U.S. ambassador was killed during an Islamist assault on the U.S. consulate there in September 2012.

A mix of militias and tribesmen has seized the main oil export ports in the east to press for political autonomy, drying up oil revenues, Libya's lifeline. (Reporting by Ulf Laessing, Ghaith Shennib and Feras Bosalum; Editing by Sophie Hares and Alister Doyle)